1. Field of the Invention
This invention is related in general to optically variable devices (OVDs) and, in particular, to vacuum-deposited polymer-based multilayer OVD structures.
2. Description of the Related Art
Optically variable devices are becoming ever more popular as tools to provide security for documents and products subject to counterfeiting, forgery, and/or diversion. Matching the proper security feature for its intended function, determining the method of the security feature's authenticity, and incorporating effective anti-counterfeiting protection for the OVD itself are all important issues faced during the design and implementation of OVDs for a specific security application. The OVD can be used as a stand-alone feature or can be combined with more conventional printed security items to create devices that are extremely difficult to replicate using photocopy or scanning technologies.
A recent development in the field has been the introduction of the OVD stripe. Generally, the stripe is an OVD continuous pattern applied at a width of about 10 mm or less. The benefit of stripes is that application speeds are very high, which reduces the unit cost of the process and makes OVD stripe application ideal for large runs, such as for banknotes. Typically, banknote stripes are designed to produce a specific color shift as the stripe is rotated with respect to a viewer (that is, by changing the angle of incidence of the light directed to the OVD). Of particular interest to the present invention are OVDs that consist of vapor-deposited etalon structures (so called Fabry-Perot structures) that produce color shifting as a result of interference effects after each wavelength is reflected from the parallel mirrors separated by the etalon spacer layer. As one skilled in the art would readily understand, such an etalon consists of multiple layers of materials, each having a complex index of refraction with real and imaginary parts that determine the reflectance, absorbance and transmittance of the layer. The materials are selected so that a light beam incident on a proximal layer of the etalon is in part reflected and in part transmitted through intermediate spacer layers to a distal layer, where a portion of the transmitted light is reflected and returned to interfere with the light reflected by the proximal layer. Those skilled in the art will also readily understand that the absorbance of the material has an affect on the color of the of the light seen by an observer of the device. Security devices produced with an evaporated monomer/polymer spacer layer are materially more flexible than conventional devices produced with inorganic spacer layers, such as MgF2, LiF, CaF, SiO2, Al2O3, etc. Such flexibility prevents crazing in applications where the OVD may be wrinkled (as in banknote applications). The lower temperature of vapor deposition also allows thinner films to be used as substrates.
It is known that the color shift produced by an etalon structure results from the phase difference between the two beams reflected by each etalon mirror after one traverses the spacer layer. In U.S. Pat. No. 6,214,422, Yializis teaches a polymeric etalon structure where the spacer layer is formed by condensation of a vapor-deposited monomer that is polymerized by exposure to radiation in vacuum. In U.S. Pat. No. 5,877,895, Shaw teaches similar color shifting structures built on a substrate with variable-thickness polymeric coatings, so as to yield different colors by changing the optical thickness of the polymers layer. This is achieved by altering process parameters such as by differentially cooling/heating the substrate and by varying the degree of cross-linking of the monomer layers.
Conventional inorganic spacer layers are deposited over the substrate as solid conformal coatings. Therefore, the spacer layer acquires a substantially uniform thickness over the roughness and imperfections of the substrate's surface. This results in a relatively uniform color shift, especially when viewed under a microscope. Spacer layers have also been deposited as organic liquid layers by vapor deposition of a monomer followed by polymerization. However, in the case of such vapor-deposited spacer layers, as illustrated in FIG. 1 with reference to a rough substrate 10 coated with a thin partially-transparent and partially-reflective metal layer 12, instead of forming a conformal coating over the substrate, the monomer is condensed as a liquid layer that wets and covers the substrate's non-uniformities (through the uniform metal layer 12), thereby producing a spacer layer 14 with a micro-rough surface 16 adhered to the metal layer 12 and a level surface 18 on the side in contact with the reflective metallic layer 20, which reduces the uniformity of the interference color shift. This effect is illustrated by the difference in the spacer-layer thickness traversed by the two wavefronts L and L′ shown in the figure. Therefore, vapor-deposited spacer layers have not yet achieved the degree of precision necessary to produce OVDs with the accurate and repeatable performance required for security applications. In spite of repeated experimentation to produce a structure with a spacer layer of precisely uniform thickness by vapor deposition, random color variations have remained an unsolved problem in the art. What is required is a product that exhibits a consistent color shift without significant variations that can be detected by the naked eye. This invention addresses this problem, particularly for the production of precision OVDs for bank notes and other security related applications.